Sputtering, alternatively called physical vapor deposition (PVD), is widely used in the commercial of semiconductor integrated circuits for depositing layers of metals and related materials. A typical DC magnetron plasma reactor 10 illustrated in cross section in FIG. 1 includes an electrically grounded vacuum chamber 12 to which a target 14 is vacuum sealed through an electrical isolator 16. A DC power supply 18 negatively biases the target 14 with respect to the chamber 12 or a grounded spatter shield within the chamber 12 to excite an argon sputter working gas into a plasma. However, it is noted that RF sputtering is also known. The positively charged argon ions are attracted to the biased target 14 and sputter material from the target 14 onto a substrate 20 supported on a pedestal 22 in opposition to the target 14. A magnetron 24 positioned in back of the target 14 projects a magnetic field parallel to the front face of the target 14 to trap electrons, thereby increasing the density of the plasma and increasing the sputtering rate. In modem sputter reactors, the magnetron may be small and be scanned about the back of the target 14. Even a large magnetron may be scanned in order to improve the uniformity of erosion and deposition.
Although aluminum, titanium, and copper targets may be formed as a single integral member, targets for sputtering other materials such as molybdenum, chromium, and indium tin oxide (ITO) are more typically formed of a sputtering layer of the material to be sputtered coated onto or bonded to a target backing plate of less expensive and more readily machinable material.
Sputter reactors were largely developed for sputtering onto substantially circular silicon wafers. Over the years, the size of silicon wafers has increased from 50mm diameters to 300mm. Sputtering targets or even their layers of sputtering material need to be somewhat larger to provide more uniform deposition across the wafer. Typically, wafer sputter targets are formed of a single circular member for some materials such as aluminum and copper or a single continuous sputter layer formed on a backing plate for more difficult materials.
In the early 1990's, sputter reactors were developed for thin film transistor (TFT) circuits formed on glass panels to be used for large displays, such as liquid crystal displays (LCDs) for use as computer monitors or television screens. The technology was later applied to other types of displays, such as plasma displays and organic semiconductors, and on other panel compositions, such as plastic and polymer. Some of the early reactors were designed for panels having a size of about 400mm×600mm. It was generally considered infeasible to form such large targets with a single continuous sputter layer. Instead, multiple tiles of sputtering materials are separately bonded to a single target backing plate. In the original sizes of flat panel targets, the tiles could be made big enough to extend across the short direction of the target so that the tiles form a one-dimensional array on the backing plate.
Because of the increasing sizes of flat panel displays being produced and the economy of scale realized when multiple displays are fabricated on a single glass panel and thereafter diced, the size of the panels has been continually increasing. Flat panel fabrication equipment is commercially available for sputtering onto panels having a minimum size of 1.8m and equipment is being contemplated for panels having sizes of 2m×2m and even larger. For such large targets, a two-dimensional tile arrangement illustrated in plan view in FIG. 2 may become necessary. Rectangular target tiles 30 are arranged in a rectangular array and bonded to a target backing plate 32.
As shown in the plan view of FIG. 2, a substantially rectangular target 30 includes rectangular target tiles 32 arranged in a rectangular array and bonded to a target backing plate 34. The tile size depends on a number of factors including ease of fabricating the tiles and they may number 4×5, but the tiles 30 may be of substantial size, for example 75mm×90mm, such that a 3×3 array is required for a larger panel. The number of tiles in the tile array may be even greater if the target material is difficult to work with, such as chromium or molybdenum. The illustrated target backing plate 34 is generally rectangularly shaped to conform to the shape and size of the panel being sputter coated but its corners 36 are rounded to conform to the chamber body supporting it and it includes an extension 38 from the chamber body containing an electrical terminal for powering the target and pipe couplings for the cooling fluid used to cool the target 30. As illustrated in cross section in FIG. 3, the target backing plate 34 for flat panel sputtering is typically formed from two metal plates 42, 44, for example, of titanium welded or otherwise bonded together. One of the plates 42, 44 is formed with linear cooling channels 46 through which the cooling fluid circulates. This backing plate 34 is more complex than the usual backing plate for wafer processing since, for the very large panel sizes, it is desired to provide a backside vacuum chamber rather than the usual cooling bath so as to minimize the differential pressure across the very large target 30.
The tiles 32 are bonded to the backing plate 34 on its chamber side with a gap 48 possibly formed between the tiles 32. Typically, the tiles 32 have a parallelopiped shape with perpendicular corners with the possible exception of beveled edges at the periphery of the tile array. The gap 32 is intended to satisfy fabricational variations and may be between 0 and 0.5mm. Neighboring tiles 32 may directly abut but should not force each other. On the other hand, the width of the gap 48 should be no more than the plasma dark space, which generally corresponds to the plasma sheath thickness and is generally somewhat greater than about 0.5mm for the usual pressures of argon working gas. Plasmas cannot form in spaces having minimum distances of less than the plasma dark space. As a result, the underlying titanium backing plate 34 is not sputtered while the tiles 32 are being sputtered.
Returning to FIG. 2, the tiles 32 are arranged within a rectangular outline 40 conforming approximately to the area of the target 30 desired to be sputtered or somewhat greater. The magnetron 24 of FIG. 1 is scanned within this outline 40. Shields or other means are used to prevent the untiled surface of the backing plate 34 from being exposed to high-density plasma and be thereby sputtered. Clearly sputtering a titanium backing plate 34 supporting molydenurn or other tiles is not desired. Even if the backing plate 34 is composed of the same material as the target tiles 32, sputtering of the backing plate 34 is not desired. The backing plate 34 is a complex structure and it is desired to refurbish it after one set of tiles 32 have been exhausted and to use it for a fresh set of tiles 32. Any sputtering of the backing plate 34 should be avoided.
The rectangular tile arrangement of FIG. 2 presents difficulties with increases in the panel size. There are several processes available for bonding target tiles to backing plates. One popular process illustrated in FIG. 4 includes an apparatus comprising two heating tables 60, 62. The tiles 32 are placed on one table 60 with their sputtering face oriented downwards. Each tile 32 is painted on its backside with a coating 64 of indium. The heating table 60 heats the coated tiles 32 to about 200° C., far above indium's melting point of 156° C. so that indium wets to the tiles 32 and forms a uniform molten layer. Similarly, the backing plate 34 is placed on the other heating table 62 and is painted with an indium coating 66 and is heated to about 200° C. With all indium coatings 64, 66 in their molten state, the tiles 32 are removed from the first table 60, inverted, and placed on top of the backing plate 34 with the melted indium coatings 64, 66 facing each other and the sputtering faces oriented upwards. Upon cooling, the indium solidifies and bonds the tiles 32 to the backing plate 34.
The transfer operation must be performed quickly enough that the indium coating 64 on the tiles 32 does not solidify during transfer. For smaller targets, the transferring could be done manually. However, with the target and tiles becoming increasingly larger, a transfer fixture grasps the edges of the tiles, and a crane lifts the fixture and moves it to the second table.
Such large mechanical structures are not easily manipulated to provide the desired degree of alignment, specifically, the bonded tiles being separated by no more than 0.5 mm. Instead, as illustrated for a corner area 40 between four tiles 32 in the plan view of FIG. 5, the four tiles 32 arranged in a rectangular array tend to slide along each other and be misaligned with different sizes for the inter-tile gaps 48. More importantly, an interstix 72 between the corners of the four tiles may become much larger than intended. By an interstix is meant a point or space at the interfaces between three or more tiles so that the term does not include the line between two tiles. Even a well defined interstix 72 presents the greatest gap between tiles 32. As a result, the widest point of the interstix 72 for misaligned tiles 32 may become larger than the plasma dark space, e.g., 1 mm, so that the plasma may propagate towards the backing plate 34. if the gap is only slight larger than the plasma dark space, the plasma state in the gap may be unsteady and result in intermittent arcing. Even if the arcing is confined to tile material, the arc is likely to ablate particles of the target material rather than atoms and create contaminant particles. If the plasma reaches the backing plate, it will be sputtered. Plate sputtering will introduce material contamination if the tiles and backing plate are of different materials. Furthermore, plate sputtering will make it difficult to reuse the backing plate for a reflurbished target. Even if the plasma does not immediately reach the backing plate, an oversized interstix 72 allows the plasma to sputter the sides of the tiles 32 facing the interstix 72. The side sputtering will further enlarge the interstix 72 and worsen the situation of plate sputtering.
A similar problem arises from the differential thermal expansion between the materials of the target tiles and the backing plate. When the bonded assembly is cooled to room temperature, the differential thermal expansion is likely to cause the assembly to bow. Because of the softness of solid indium, the bow can be pressed out of the bonded assembly. However, the pressing is a generally uncontrolled process and the tiles may slide relative to each other during the pressing to create the undesired tile arrangement of FIG. 5.
Techniques have been developed to bond tiles to backing plates with a conductive elastomer that can be applied at a much lower temperature. Such bonding services are available from Thermal Conductive Bonding, Inc. of San Jose, Calif. Nonetheless, elastomeric bonding does not completely eliminate the misalignment problem with larger array of target tiles.